U-Pb zircon geochronology of rocks in the Salinas Valley region of California: A reevaluation of the crustal structure and origin of the Salinian block

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature melange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 4 0 Ar/ 3 9 Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite 4 0 Ar/ 3 9 Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona.